Ureteral peristalsis in five pregnant patients with signs of acute renal outflow obstruction was studied with the aid of a new technique for recording intraureteral pressure (ureterometry). With this technique it is possible to record the intraureteral pressure at three different levels in the ureter simultaneously. The rate of propagation and direction of the peristaltic waves can thus be determined. Retrograd peristalsis was found in three of the five patients and in one patient and in one patient no peristaltic activity was present at all. The fifth patient demonstrated normal antegrade peristalsis. The findings at renography in the acute stage were in accordance with the ureterometric data.